1. Field of the Invention
The present invention relates to a lens shading correction device and method in an image sensor.
2. Description of the Background Art
An image sensor, a device for photographing an image using the properties of semiconductor reacting to light, is a device for sensing, by a pixel, brightness and wavelength of each different light emitted from respective objects and converting the sensed light into an electrical value. The function of the image sensor is to convert the electrical value into a level for signal processing.
In detail, the image sensor is a semiconductor device for converting an optical image into an electrical signal. Of the image sensors, Charge Coupled Device (CCD) is a device in which Metal Oxide Semiconductor (MOS) capacitors are located very close to each other and store and transfer charges, respectively. A Complementary Metal Oxide Semiconductor (CMOS) image sensor uses a CMOS technology in which a control circuit and a signal processing circuit are used as a peripheral circuit and employs a switching mechanism in which an output is detected sequentially using MOS transistors provided as many as the number of pixels.
The CMOS image sensor is of great use to a personal mobile system such as a mobile phone because of a great advantage of low power consumption. Thus, the CMOS image sensor is diversely applicable to a Personal Computer (PC) camera, for use in medical science, a toy, etc.
In the image sensor generally photographing an optical image of an object through a lens, there occurs a lens-shading phenomenon in which a brightness of an edge region of a photographed image gets lower than a brightness of a central region under the influence of a convex lens shape.
FIG. 1 illustrates the lens-shading phenomenon. Referring to FIG. 1, as a lens transmits light, an edge region of the lens is reduced in brightness compared to a central region.
If light in which the edge region is reduced in brightness is incident on the image sensor, an edge region of a displayed image is reduced in brightness. A magnitude of lens shading is dependent on a wavelength of light emitted from an object.
FIG. 2 illustrates a lens-shading phenomenon based on R, G, and B (Red, Green and Blue) color information.
Referring to FIG. 2, as a lens transmits a light emitted from an object, an edge region of the lens is reduced in brightness compared to a central region. A degree of brightness reduction is different depending on Red (R), Green (G), and Blue (B) color information considering wavelength information on light.
If light in which a magnitude of brightness reduction at the edge region is different depending on the R, G, and B color information that is incident on the image sensor, there occur drawbacks as follows.
(1) The edge region of the displayed image is reduced in brightness.
(2) The expression of gray level based on the R, G, and B color information is distorted in an image region corresponding to the edge region of the lens, thereby making it impossible to display accurate color and increasing a lattice noise. This is in contrast to the expression of gray level based on the R, G, and B color information that can be made without distortion in an image region corresponding to the central region of the lens.
Tables 1 and 2 are provided below to describe the lattice noise.
TABLE 1Brightness based on Gr and Gb colorinformation in image region corresponding to central region of lens126127126129129128128127129130126127126
TABLE 2Brightness based on Gr and Gb colorinformation in image region corresponding to edge region of lens126127126148150128128127149151126127126
As known from Tables 1 and 2, there does not occur a lattice noise in the image region corresponding to the central region of the lens because brightness based on Gr and Gb color information are almost consistent, but there occurs a lattice noise in the image region corresponding to the edge region of the lens because there is a great difference between brightness based on Gr and Gb color information.
A magnitude of lens shading is different depending on an object photographing environment, that is, depending on brightness.
If light in which a magnitude of brightness reduction at the edge region is different depending on the R, G, and B color information is incident on the image sensor, there occur drawbacks as follows.
(1) The edge region of the displayed image is reduced in brightness.
(2) The expression of gray level based on the R, G, and B color information is distorted in the image region corresponding to the edge region of the lens, thereby making it impossible to display accurate color. This is in contrast to the expression of gray level based on the R, G, and B color information that can be made without distortion in the image region corresponding to the central region of the lens.
(3) The expression of gray level based on the R, G, and B color information is distorted in the image region corresponding to the edge region of the lens because of a change of a brightness environment, thereby making it impossible to display accurate color in the edge region.
Such a lens-shading phenomenon is an important cause of a reduction of a quality of the image sensor.